Ohmic heating is a well-known method for heating liquids. A pair of electrodes is used to apply a voltage or an electrical field and generate an electrical current directly through the liquid medium to be treated. This technique can be applied to liquids containing free ions, which are therefore electrical conductors. The movements of the electrical charges create heat in the product according to Joule's law. Heat generation is a direct function of geometry, the electrical conductivity of a given product, and voltage. Ohmic heating is a volume heating, as opposed to heating by convection from the hot surface of a heat exchanger.
When this method is applied to a fluid circulating in a duct to implement a continuous heating process, the liquid circulating at low speed close to the walls of the duct reaches higher temperature than the liquid circulating at higher speed at the centre of the duct because of its longer exposition to voltage. Hence, overheating of the liquid may be experienced close to the walls due to the non-uniform profile of speeds in a cross-section of the duct, which is due to the viscosity of the liquid. The overheating of the liquid may in turn result in detrimental effects such as coagulation, fouling and/or clogging and to the interruption of the production line.
Document FR 2 863 829 discloses a continuous heat treatment of a fluid that is heated via electrodes spaced along the longitudinal flowing axis. The surface temperature of the fluid is controlled thanks to temperature sensors associated with an outside cooling flow. By continuously lowering the surface temperature, this concept decreases pipe clogging. However, a cooling fluid is needed to cool the main fluid, and an additional system comprising pumps, pipes and other devices is also needed to handle that cooling fluid. As a result, the system is complex, costly and requires an increased maintenance. In addition, the energy transferred to the cooling fluid is lost.